1. Field of the Invention
The present invention relates to an integrated optical device having a substrate and optical waveguides disposed thereon and a method for manufacturing such a device. More particularly, the present invention relates to an integrated optical device so structured as to exhibit a desired polarization dependence or independence by adjusting the birefringence of optical waveguides of the device and to a method for manufacturing the device.
2. Description of the Prior Art
A single-mode channel optical waveguide formed on a planar substrate, in particular a silica-based single-mode channel optical waveguide formed on a silicon substrate, is described in, for instance, the article by N. Takato et al. entitled "Guided-Wave Multi/Demultiplexer for Optical FDM Transmission" (Technical Digest of ECOC '86, p. 443). In such a silica-based single-mode channel optical waveguide, the size of the cross section of the core portion thereof can be established at 5 to 10 .mu.m which is consistent with that of a commonly used silica-based single-mode channel optical fiber. Therefore, it is expected that the silica-based single-mode optical waveguide can be used as a means for obtaining practically useful integrated optical devices which are excellent in matching properties with an optical fiber.
FIGS. 14A and 14B are, respectively, a plan view and an enlarged sectional view taken along line A-A' in FIG. 14A, illustrating a configuration of a guided-wave Mach-Zehnder interferometer as an example of such a conventional integrated optical device in which this kind of silica-based single-mode channel optical waveguide is employed.
In FIGS. 14A and 14B, reference numeral 1 denotes a silicon substrate. Reference numerals 2 and 3 denote directional couplers formed, on the silicon substrate 1, from a silica-based glass material. The directional couplers 2 and 3 are comprised, respectively, of a pair of two silica-based single-mode channel optical waveguides 2-1 and 2-2, and 3-1 and 3-2 adjacent to one another, and each coupling factor thereof is established to be about 50%. Reference numerals 4 and 5 represent two optical waveguides. The optical waveguide 4 connects the optical waveguides 2-1 and 3-1 of the directional couplers 2 and 3 to each other. The optical waveguide 5 connects the optical waveguides 2-2 and 3-2 of the direction couplers 2 and 3. The optical waveguide 4 is longer, by .DELTA.L, than the optical waveguide 5. Each of these optical waveguides 2-1, 2-2, 3-1, 3-2, 4 and 5 is comprised of a core glass portion embedded in a cladding glass layer 12 disposed on the substrate 1. The optical waveguides 2-1 and 2-2 are provided with input ports 1a and 2a and the optical waveguides 3-1 and 3-2 are provided with output ports 1b and 2b. In addition, a thin film heater 6 is disposed on the cladding glass 12 and above the optical waveguide 5.
In this optical device, it is known that optical signals can alternatively be outputted from the output ports 1b and 2b with a period (.DELTA.f) represented by the following formula: EQU .DELTA.f=c/2n.DELTA.L
(wherein c represents light velocity and n is the refractive index of the optical waveguide)
, as the frequency of the optical signal inputted through the input port 1a is varied.
On the other hand, FIG. 15 shows the periodic characteristics of the interferometer shown in FIGS. 14A and 14B. That is, FIG. 15 illustrates the frequency characteristics of the output light from the output ports 1b and 2b observed when a TE polarized wave having a polarization direction parallel to the substrate 1 is inputted as a signal light to the input port 1a. In FIG. 5, the solid line curve represents the frequency characteristics of the output light from the output port 1b and the broken line curve represents the frequency characteristics of the output light from the output port 2b.
In this respect, it is, for instance, assumed that two optical signals f.sub.1 and f.sub.2 having a frequency difference (.DELTA.f) therebetween of 10 GHz are simultaneously inputted through the input port 1a in the band of 1.55 .mu.m. If .DELTA.L in the aforementioned formula is established at about 10 mm, the two optical signals f.sub.1 and f.sub.2 can separately be outputted from the output ports 1b and 2b. In practice, the thin film heater 6, which serves as a phase shifter for changing the effective optical path of the optical waveguide 5 by approximately one wavelength in accordance with the thermo-optical effect, is disposed on one optical waveguide 5 and the electric voltage applied to the thin film heater 6 is so adjusted to synchronize the foregoing period of the guided-wave Mach-Zehnder interferometer with the frequencies of the optical signals f.sub.1 and f.sub.2 and to output a desired optical signal from a desired output port, whereby the Mach-Zender interferometer shown in FIGS. 14A and 14B as a whole functions as a optical frequency division multi/demultiplexer.
Such a device can be manufactured in accordance with a conventional method which mainly utilizes the techniques for depositing a silica-based glass film onto a silicon substrate and for finely processing the resultant glass film by reactive ion etching.
However, the guided-wave Mach-Zehnder interferometer shown in FIGS. 14A and 14B is affected by a strong compression stress generated within the surface of the glass film due to the difference of the thermal expansion coefficient between the silica-based glass and the silicon substrate. As a result, the optical waveguide exhibits a stress-induced birefringence of the order of B.sub.O =5.times.10.sup.-4 (expressed in the value of birefringence). Here, B.sub.O =n.sub.TM -n.sub.TE, wherein n.sub.TE is the effective refractive index of a TE polarized wave and n.sub.TM is the effective refractive index of a TM polarized wave. In other words, the effective refractive indexes n (i.e., n.sub.TE and n.sub.TM) differ from one another by B.sub.O depending on the polarization direction of the incident light. This causes a deviation in the phase of the above-described periodic characteristics of the interferometer depending on the polarization directions. As a result, there is the problem that the interferometer does not function as an optical frequency division multi/demultiplexer at all, unless the polarization direction of an optical signal is previously adjusted to a direction either parallel (TE polarized wave) to or vertical to (TM polarized wave) the surface of the substrate.
It is known that if the value of the birefringence of the optical waveguide constituting an interferometer such as shown in FIGS. 14A and 14B can be freely controlled, the guided-wave interferometer can be so designed that the periodic characteristics of the TE polarized wave apparently coincide with those of the TM polarized wave. Namely, such an interferometer may be obtained by adjusting the birefringence of the optical waveguide, so that a slight difference (B.sub.O .multidot..DELTA.L) in the difference between the optical paths (.DELTA.L) of the interferometer due to the polarization direction is made equal to m times the wavelength of the signal light (wherein m is an integer inclusive of zero).
In the conventional methods for manufacturing integrated optical devices, however, the only method for controlling the birefringence is to change the composition of the glass used for making waveguides or the kind of substrate. This causes a problem when sophisticated integrated optical devices are constructed. Alternatively, the value of the birefringence of an optical waveguide can be varied by changing the cross sectional shape of its core portion from square to a longitudinally or transversely elongated rectangular shape to make use of the shape effect. However, the variation in the birefringence in this case is only of the order of 10.sup.-5 and this variation is not practically sufficient. In this case, if the cross sectional shape is an extreme rectangular shape, there is the problem that connection losses when the input and output ports are connected to optical fibers are extremely increased.
In order to eliminate the above-described problems of the guided-wave interferometer, an interferometer provided with one or more grooves for adjusting a stress-induced birefringence is proposed, for instance, in the article by M. Kawachi et al., entitled "Birefringence control in high-silica single-mode channel waveguides on silicon" (Technical Digest of OFC/IOOC '87, TUQ31) or European patent application Laid-Open No. EP-0255270-A2.
FIGS. 16A and 16B are, respectively, a plan view and an enlarged cross sectional view taken along line A-A' of FIG. 16A, both showing an example of the configuration of such an interferometer provided with grooves for adjusting the stress-induced birefringence. This example differs from that shown in FIGS. 14A and 14B in that the former has stress adjusting grooves 21a and 21b for controlling the stress-induced birefringence of the optical waveguide by releasing a part of the stress from the substrate 1. The stress adjusting grooves 21a and 21b are disposed on the cladding glass layer 12 adjacent to the core portion of the optical waveguide 4 and can be formed by a reactive ion etching process. In principle, it seems that the dependence of the interferometer upon the inputted polarized wave can certainly be eliminated by establishing the position, depth, width and length of the grooves 21a and 21b, so that the slight difference in the difference .DELTA.L between the optical paths of the interferometer due to the polarization directions is made equal to m times the wavelength of the signal light (m is an integer). This structure, however, has the following problem concerning the manufacture thereof.
When an integrated optical device having such a construction as that shown in FIGS. 16A and 16B is manufactured, the stress adjusting grooves 21a and 21b are formed by removing a part of the cladding layer 12 by a reactive ion etching process. It is, however, impossible to simultaneously measure (perform on-line monitoring) the polarization characteristics of the interferometer during the ion etching process. This is because the reactive ion etching process is effected using plasma within a vacuum chamber and, therefore, it is difficult to form such a groove while monitoring the polarization characteristics by introducing a monitoring light into the processed interferometer. As a consequence, the processed device is sometimes etched excessively and thus it is difficult to obtain an interferometer exactly turned to desired polarization characteristics.
There has been proposed a further method in which a stress applying portion is formed in the cladding layer 12 in the vicinity of the core portion of the optical waveguide instead of forming the stress adjusting grooves 21a and 21b shown in FIGS. 16A and 16B, so that the value of birefringence in a desired part of the optical waveguide is adjusted. FIG. 17 is a cross sectional view showing an example of such a conventional optical waveguide provided with such stress applying portions. In accordance with this construction, the stress applying portions 22a and 22b of polycrystalline silicon are arranged on both sides of the core portion 4 and in the vicinity thereof to adjust the birefringence of the optical waveguide 4. In order to exactly adjust the birefringence at a desired value according to this method, the shape, position and required length of the stress applying portions 22a and 22b must be determined on the basis of an accurate calculation of stress distribution. In accordance with the results, a glass film and a silicon film must be deposited and etched, without any error, to form an optical waveguide structure with the desired stress applying portions thereon. However, such procedures can be carried out only with great difficulties concerning processing.
While the importance of the birefringence adjustment of optical waveguides in the manufacture of integrated optical devices and the problems associated with the conventional methods for producing the devices have been discussed with respect to the dependence of a guided-wave Mach-Zehnder interferometer upon the inputted polarized wave as an example, the same problems likewise arise when manufacturing other integrated optical devices such as optical ring resonators, Fabry-Perot resonators, polarization beam splitters, mode converters, wave plates, directional coupler and so on.
In the case of fabricating integrated optical devices provided with a single-mode optical waveguide formed from materials other than the material for a silica-based single-mode optical waveguide, it is also desired to adjust the birefringence of the optical waveguide to produce devices exhibiting desired polarization characteristics. For example, in integrated optical devices essentially provided with an ion-diffused glass waveguide in which a core portion is formed by diffusing ions capable of increasing the refractive index of the multi-component glass into desired portions on the surface of a substrate composed of the multi-component glass, it is known that the core portion of the optical waveguide is affected by stress from the substrate and thereby a stress-induced birefringence is created therein. In order to impart a desired polarization dependence to an integrated optical device, it is, of course, necessary to precisely control the stress-induced birefringence. There has, however, never been known a method for precisely tuning a value of the birefringence, while allowing errors associated with the fabrication of the optical waveguide.
In addition to the glass type integrated optical devices discussed above, it is also very important to precisely control the birefringence value of optical waveguides when fabricating integrated optical devices in which optical waveguides of LiNbO.sub.3 type materials; optical waveguides of semiconductors such as InP or GaAs; and optical waveguides of magnetic materials such as YIG type materials are used. For instance, an attempt has been made to fabricate an integrated optical isolator composed of a GGG substrate and a YIG type optical waveguide formed thereon. However, it is necessary that the stress-induced birefringence of the YIG type optical waveguide due to the stress from the GGG substrate be restricted to zero in order to ensure smooth Farady rotation of the polarization plane while the optical signal is transmitted through the YIG type optical waveguide.
For this purpose, a method for forming a silica glass film serving as stress-induced portions on the YIG type optical waveguide has been proposed. However, it is difficult to precisely control the birefringence value of the resulting waveguide for the same reasons as discussed above in connection with the conventional embodiments shown in FIGS. 16A, 16B and 17.
Alternatively, a method for cancelling the stress from the GGG substrate by applying a weight to the upper portion of the YIG optical waveguide has been proposed. However, there is a high possibility that the GGG substrate per se will be damaged by the application of the weight. In addition, such a solution has not been practically acceptable when a variety of optical elements are integrated on a single substrate.
The aforementioned drawbacks associated with the conventional methods for fabricating integrated optical devices, i.e., the lack of precise and easy control of the birefringence value of the optical waveguide is a serious obstacle in designing and fabricating integrated optical devices such as interferometers, a ring resonators, polarization beam splitters, isolators and so on, in which the birefringence characteristics of the optical waveguide play an important role.